The methods of electrical energy transformation into the force and shift are various. The actuators in microelectronics are based on the transducers properties of new or improved solid-state materials. They may offer advantages over conventional electromechanical transducers in that they lack moving parts. The input of the actuator should be driven electrically with low losses, currents, and voltages that are compatible with microelectronics whenever possible. The low voltages and currents also meet the requirements of low interference of the induced electrical or magnetic fields. However, the actuators have to induce needful and often high mechanical forces and large displacements at low voltages and currents. So, the most important aim is the design of the smallest possible actuators driven by low voltages and currents with low losses and high enough output of mechanical work. Mere size reduction of the classical engineering designs has only limited the potential and is incorrect in essence. New principles, materials and technologies head towards the new and acceptable actuators. Tab. 1 presents the main features of contemporary actuators in comparison with the new sliding nanoactuators (SNA, patent application).
TABLE 1Contemporary actuators and SNA (patent application).ForceEnergyResponseWattagePositioningInputDeformationdensitydensitytimedensityaccuracyvoltageGroupParagraphPrinciple(%)(Pa)(J/m3, Pa)(s)(W/m3)(m)(V)Practical2.1Piezoelectric0.35E71E51E−5 1E101E−9100applicationactuators2.2Magnetostrictive0.35E71E41E−51E91E−810actuators2.3Shape memory107E84E61 4E61E−410actuators[1:155]Theory/2.4Electronic101E61E51E−31E71E−4100Experimentspolymers2.5Ionic101E51E40.11E51E−41polymers[2:161][2:161]2.6Conductive33E61E51 1E51E−410polymers[2:250]2.7Carbon nanotubes15E93E71E−6 3E101E−91[2:277][2:277][2:278][2:278]2.8Molecular305E51E41E−31E61E−90.1actuatorsNatural2.91 twitch34E51E31E−31E61E−40.1muscle[2:161]2.9100 twitches3004E51E50.11E61E−40.1SNA nanoactuator3Hydrocarbon - Water*,35E51E41E−9 1E131E−90.1(proposal)1 twitch3Hydrocarbon - Water*,3005E51E61E−7 1E131E−90.1100 twitches3Carbon - Water*,3001E71E81E−7 1E151E−90.1100 twitchesParameters represent the best reached/expected values.*materials of functional nanoparticles - medium
Piezoelectric Actuators
The piezoelectric effect is based on the elastic deformation and orientation of electric dipoles in a crystal structure. Applying an electrical field causes the deformation of the dipoles, leading to a strain on the crystal. Many current materials used are based on alloys of lead, zirconate and titanate (PZT ceramics) or piezoelectric films on the basis of polyvinylidenfluoride (PVDF). They have been used successfully for many years to solve demanding problems in the field of precision positioning and active vibration control.
Piezoelectric actuators are characterized by high output actuation forces, short reaction times (or high operating frequency), and positioning accuracy of the order of a few nanometers—but only at low strains. Contemporary, rather high voltages are not directly compatible with voltages in microelectronics.
Magnetostrictive Actuators
Magnetostriction occurs in most ferromagnetic materials. Rare-earth-iron (Tb—Dy—Fe) “Giant Magnetostrictive Alloys” (GMAs) feature magnetostrains that are two orders of magnitude larger than ferromagnetic elements such as Nickel. The static magnetostrain of the GMAs permits the building of linear actuators offering small displacements and large forces.
Magnetoresistive tranducers have some specific advantages over piezoelectric actuators—low hysteresis, higher operating temperature, lower voltage driving signal, the possibility of current control and the possibility to separate the driving coil from the magnetoresistive rod. Although piezoelectric and magnetostrictive transducers are exchangeable in many applications—piezoelectric transducers have established a greater presence, particularly because of the greater variety of commercially available piezoelectric ceramics and ready-made actuators.
Shape Memory Actuators
The term shape memory (SM) refers to the ability of certain materials to recover a predefined shape. The SM effect is based on a solid-solid phase transition of the shape memory alloy that takes place within a specific temperature interval. Above the transition temperature, the crystal structure takes on the so-called austenitic state. The martensitic crystalline structure will be more stable for thermodynamic resons if the temperature of the material drops below the transition temperature. Boundaries of twinned martensite can easily be moved; for that reason SM elements can be deformed with quite low forces in the martensitic state. When heated up, the austenitic structure will be established again. At the same time, the SM material will return to its original shape. Nickel-titanium (NiTi) and nickel-titanium-copper (NiTiCu) have the best properties for actuator purposes. Its transformation is limited to approximately 100° C.
The element may exert high forces when recovering its predefined shape. Heating up the SM element is relatively simple. When conducting an electrical current, heat is generated because of Joule losses. During the cooling process, rather slow spontaneous processes remove heat. However, small actuators offer a much higher surface-to-volume ratio. Hence, heat transfer to the surrounding medium is strongly improved, resulting in faster response times of the actuators. Sputtering is a typical fabrication process employed. The very high work-per-volume ratio is highly valued if the space is limited. The disadvantage of low efficiency below 2% is determined by thermodynamics. Because of the low efficiency and low speed, electrical shape memory actuators offer a good choice for very special applications.
Electronic Polymers
Ferroelectric polymers, such as polyvinylidenfluoride (PVDF), exhibit spontaneous electric polarization. A large applied alternating current field (˜200 MV/m) can induce electrostrictive (nonlinear] strains of nearly 2%.
Electrets are polymers that retain their electric polarization after being subjected to a strong electric field. Current applications of electrets include electrostatic microphones.
Dielectric EAP, also known as electrostatically stricted polymers can be represented by a parallel plate capacitor. The observed response of the film is caused primarily by the interaction between the electrostatic charges on the electrodes (Maxwell effect). The opposite charges on the two electrodes attract each other, while the identical charges on each electrode repel each other.
An electrostrictive graft polymer consists of two components, a flexible backbone macromolecule and a grafted polymer that can form a crystalline structure. The grafted crystalline polar phase provides moieties in response to an applied electric field and cross-linking sites for the elastomer system. This material offers a high electric-field-induced strain ˜4%.
Liquid-crystal elastomer materials can be electrically activated by inducing Joule heating. The actuation mechanism of these materials involves the phase transition between nematic and isotropic phases over a period of less than a second. The reverse process is slower, taking about 10 seconds.
Ionic Polymers
Ionic Polymer Gel (IPG) is a solution or colloidal suspension that undergoes a physical or chemical change to a solid while retaining much of the solvent within the structure. The expansion and contraction of gels depend on the diffusion of water or solvent in and out of the matrix. The response time of a 1-mm thick sheet responds in about 20 minutes, 30-μm contractile bodies have been found in plants that contract in response to calcium ions in about 50 ms [2].
Ionic Polymer-Metal Composites (IPMC) form a subgroup of IPG. They consist typically of a thin (about 200 μm) polymer membrane with thin metal electrodes (about 10 μm). The polyelectrolyte matrix is neutralized with an amount of counter-cations, balancing the charge of anions covalently fixed to the membrane (e.g. Nafion, DuPont, neutralized with alkali metals). When an IPMC in the dissolved (i.e. hydrated) state is stimulated with a suddenly applied step potential (1-3 V), mobile counter-cations diffuse toward the cathode. As a result, the composite undergoes an initial fast bending, followed by a slow relaxation.
Conductive Polymers
Conductive polymers are chemically characterized by the so-called conjugation in which carbon double bonds periodically alternate with carbon single bonds along the polymer backbone (polypyrole, polyaniline, polyacetylene). Conductive polymers have a rather high electrical conductivity when doped with ions. Unlike in silicone, the dopants can be easily inserted and removed from the spaces they occupy between the polymer chains. In comparison with other semiconducting materials, the doping level can be very high: approximately one dopant counterion per four monomers. Construction of the actuators is possible due to the volume changes that take place in these materials during the changes in the doping level.
For instance, when a polypyrol film is grown electrochemically onto a positively charged electrode, the film will be automatically p-doped (FIG. 1A, electrons are removed, the polymer is oxidized). As the electrolyte, salt sodium dodecylbenzene sulfonate (NaDBS) can be used [1, p. 211]. When the salt is dissociated to Na+ and DBS−, the anions DBS− are built into the doped polypyrole network to maintain the charge neutrality of the polypyrol film. Because of their large size, DBS− anions are immobile and cannot diffuse easily out of the polymer. Polypyrol remains stable in the p-doped state when no external potential is applied. If a negative potential is applied to the electrode (FIG. 1B), electrons flow back into the polymer and satisfy the missing charges (electrons are accepted, the polymer is reduced). The cations Na+ simultaneously enter the polymer to compensate for the immobile anions, forming the salt NaDBS. Macroscopically the polymer increases its volume. Depending on the magnitude of applied voltage, polypyrol can change the volume by up to 2%. If a positive potential is applied (FIG. 1C), then the electrons and cations move outside the polymer again, the volume of the polymer decreases to the state on FIG. 1D equivalent to FIG. 1A. So, the ionic concentration and hence the volume of polymer may be promptly controlled by an external electrical field. In general, the following three effects can be responsible for the volume changes in conductive polymers:                1) insertions of counterions (most dominant in the mentioned example of polypyrole);        2) conformations of chains (e.g. straight or twisted chains);        3) interactions between chains (e.g. electrostatic forces);        
The most studied conductive polymer actuators are based on a “conductive polymer film/no volume change film”, where the conductive polymer is the only electromechanically active material. The conductive polymer film is connected as a working-electrode in an electromechanical cell; an additional counter-electrode and an electrolytic solution are necessary.
The actuation mechanism of a conductive polymer actuator is based on an ionic exchange between the conductive polymer film and the electrolytic medium (either an electrolytic solution or a dry/wet polymer electrolyte). This is the most important factor that controls and limits the rather long response time of this kind of actuator. The low linear strain of the conductive polymer impedes the construction of the effective linear actuators working in the same way as natural muscles. This fact is an important limitation that has to be overcome looking for either new polymers or a new configuration where longitudinal changes should be amplified.
Carbon Nanotubes
Carbon nanotubes have exciting electrical and mechanical properties derived from their structure that consist of hollow cylinders of covalently bonded carbon. Double-layer charge injection seems to be the most promising actuation mechanism [2, p. 262]. This mechanism is quite distinct from that observed for conducting polymers. The ion flows, and consequently the electromechanical actuation behavior can be quite complex and is determined by the relative mobility of the polymer counter-ion and the ions in the electrolyte. In contrast, the carbon nanotube acts as an electrochemical capacitor with the charge injected into the nanotube balanced by the electrical double-layer formed by movement of the electrolyte ions to the nanotube surface. The charge injection causes quantum chemically based dimensional changes in the carbon-carbon covalent bond length of the surface atoms close to the double layer. Short response times and high force density are expected from this mechanism based on the function of strong covalent bonds. However, the problem with a small low linear strain remains to be solved.
Electrostatic repulsion between different tubes in the nanotube “forest” would cause repulsion between these tubes. An inter-tube electrostatic actuator mechanism may also be scientifically interesting.
Molecular Actuators
The stimulus-induced conformational change within the single molecule is the foundation of most biological molecular actuators. An example of the molecular actuator is the transport system powered by the motor protein kinesin [5]. The random coil-to-helix transformation is among the most powerful conformational changes [2, p. 304]. The common principal disadvantage of biomolecular motors is their limited lifetime in vitro, and the narrow range of environmental conditions that they are able to tolerate.
Natural Muscles
Mammalian skeletal muscles are considered highly optimized systems evolved over more than 600 million years. They are made up of individual muscle fibers—single cells, which are up to several centimeters long, cylindrical with diameters about 10 μm, and surrounded by a cell membrane. The muscle cells are made up of parallel myofibrils. Myofibrils are built up from several micrometer axial long contractile units called sarcomeres, which contain three filament types: thin, thick and connecting (FIG. 2). Thick (myosin) and thin (actin) protein filaments inside the sarcomere are recognized to play a central role in contraction. The actin and myosin proteins have a contractile mechanism, which can be interpreted as both sliding and folding [3].
(1) The sliding mechanism has become broadly accepted since about 1960. When the muscle cell is electrically excited by action potential, transmitted along their cell membrane by nerves, calcium ions are released from the sarcoplasmic reticulum (it is placed near to the Z-plates) and bind to the specific sites of the actin filaments. The closest myosin head groups then bind these sites and the thick and thin filaments seem to be mechanically connected, but no movement has occurred. Movement requires the head groups to change their angle and drag the thick and thin filaments past one another. Energy is needed for this process and is provided in the form of ATP. The head groups possess adenosine triphosphatase (ATPase) enzymatic sites that are active only when the heads are combined with actin. The active complex in the presence of Mg2+ hydrolyzes ATP into inorganic phosphate and adenosin diphosphate (ADP). Part of the energy released is used to change the position of the head groups from extension to flexion. Unfortunately, scientific explanations of the swinging mechanism of the heads are rather confused. This process is repeated, and the thin filament is pulled toward the middle of the sarcomere and the sarcomere is shortened. When electric activity ceases, excess calcium is rapidly taken up by the sarcoplasmic reticulum. The rest of the ATP-ADP splitting energy is consumed when the calcium ions return to the sarcoplasmatic reticulum. Without the bound calcium, the head groups cannot remain bound to actin and without another timely electrical excitation, the sarcomere lengthens, and the muscle once again relaxes. However, with a sufficient high frequency repetition of electrical excitation, the sarcomere can contract up by individual twitches to its minimal length. The mechanism explains many known features of contraction.
(2) Folding mechanism. There are several reasons for considering the sliding mechanism as inadequate. Until the mid-1950's, muscle contraction was held to occur by a mechanism similar to protein folding, resembling a phase-transition. Also recently, Pollack [3] suggests that much of cellular biological functions may be governed by a single unifying mechanism—phase-transition. The sarcomere contraction is one of the main examples. It is supposed to be the result of the calcium induced phase transitions in all three filaments: condensation in thin filaments, folded—unfolded state transition in the connecting filaments, and helix to random coil phase transition of the myosin filament. However, several questions remain unanswered.
(3) Phase-transition of lattice of myosin heads answers the existing questions [6]. It includes both point (1) and (2) of the view of the interpretations. Van der Waal's attraction during the calcium-induced phase-transition of the array of myosin heads (reconstruction of their arrangement amongst themselves) represents the primary driving force. A simple lever mechanism, which converts the van der Waal's approaching of myosin heads into the thick and thin filaments sliding sets up the gearing mechanism.
The alternative model (3) also represents the theoretical background of the patent application.